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Molecular mobility in polyaniline studied by ESR method
ELSEVIER
Synthetic
Metals
69 (1995) 223-224
Molecular mobility in polyaniline
studied by ESR method
A.V. Kulikov, Ya.L. Kogan, L.S. Fokeeva Institute of Chemical Physics in Chemogolovka, Chemogolovka, Moscow region,
Russian Academy 142432 Russia
of Sciences,
Abstract For emeraldine salts of polyaniline (ES), segmental motions in precipitated ES and large-scale rearrangements of ES globules in solutions of polyaniline in N-methylpyrrolidinone (NMP) were found, in spite of the formation of salt bridges between polyaniline chains. For precipitated ES it was found that the asymmetry of the Dysonian ESR line correlates with macroviscosity of solvent. This observation reveals the important role of segmental motions of polyaniline chains in conductivity of ES at microwave frequeucy. For solutions of ES in NMP the broadening of the ESR lines at room temperature and 77 K was found at dilution of the solutions. This observation shows the increase of the distances between paramagnetic centres of ES with the dilution. INTRODUCTION
parameter
The doped conducting form of polyaniline, emeraldine salt (ES), has many salt bridges and is considered in theory of polyaniline conductivity as a solid with static exchange interactions between neighbour chains (see, for example, [l]). Some experimental observations prove that indeed ES is more rigid than undoped emeraldine base (EB). For instance, EB is dissolved in solvents more easily than ES; slow structural relaxation occurs in polyaniline only when it is kept at a negative potential in the undoped form [2]. Nevertheless, in the present report we show that ES, both in powder and dissolved forms, reveals a substantial molecular dynamics. MATERIALS
AND DTSCUSSION
The ESR spectra of ES powders are single lines asymmetric due to the effect of conductivity of samples (the Dysonian lines). The temperature dependences of the asymmetry
0379~6779/95/$09.50
SSDI
in Fig.
1 [3]. The value
of A/B
1 t
25 t
AND METHODS
ES was synthesised during 5 h at decreased temperature in acetonitrile reaction mixture containing 1 M of aniline and 1 M of HC104 at gradual addition of 1 M @B&)&Or. Precipitates were washed several times by acetonitrile and water, then washed by 0.1 M HC104 (to produce salt form of polyaniline ES) or by 0.1 M NaOH (to produce base form of polyaniline EB), and finally dried at 40’ C. ESR spectra were recorded with a SE/x-2544 Radiopan spectrometer equipped by a temperature controller type 660. The ESR line width, AH, was measured between extreme of the first derivative. The asymmetry parameter, A/B, is the ratio of positive and negative peaks (measured as deviations from the background line) of the first derivative. The number of electrons per ring was calculated from the second integrals of ESR spectra of ES and a standard sample. The factor of line shape A/S is the ratio of amplitude and second integral. RESULTS
3.0
A53 are shown
0
1995 Elsevier Science S.A.
0379-6779(94)02425-X
All rights resewed
Fig. 1. Temperature dependence of the asymmetry parameter of ESR lines for powders of ES in different solvents. (1) dry powder, (2) water, (3) mixture of glycerol and water, 1: 1, (4) 1 M HCl, (5) 0.1 M HC104
depends on conductivity (at microwave frequency, in our case’ 9.2 GHz) and size of samples. We used samples of the same size, ca. 1x1~2 mm. For dry powder (curve 1) A/B=1 at all temperatures, i.e. the conductivity of this sample is small. The wetting of the ES powder in some solvents leads to the increase of the parameter A/B, i.e. to the increase of the conductivity (curves 2 - 5). ln our opinion, the solvents facilitate molecular motions of ES (by breaking hydrogen bonds) that results in more frequent collisions between neighbouring chains and, thus, in hopping polarons and increased conductivity. The sharp changes of A03 at freezing are explained by formation of ice that slows down molecular motions in ES.
224
A. V. KuIikov et al. / Synthetic Metals 69 (1995) 223-224
Ihe results of titration of 1% solution of EB in NMP with formic acid are shown in Fig. 2. The line width is actually constant, therefore the variations of the factor of line shape A/S are explained by the superposition of a broad line, the amplitude of the broad line being maximum at 0.05 ml/ml. The measurements of the second moment contirm the existence of a broad line at low volumes of added formic acid. 0.4
o-o-.
E
A\
0
-0.2
-
.-O-O-O o-o-o-o-o-o+
/o
0.0-O
g
\0.
of the amplitude of the broad line to that of narrow one has maximum at 0.05 ml/ml, we would measure a minimum of the second integral at this concentration of formic acid. The ESR spectrum of solutions of polyaniline in Nmethylpyrrolidinone doped with formic acid at 0.2 ml/ml is a Lorentzian line. The line width increases with dilution both at 77 and 293 K (Fig. 3). In this experiment the concentration of formic acid was maintained constant. The width of the ESR line of ES is determined by the averaging of the hyperline structure by static or dynamic exchange interactions between polarons. We explain the broadening by the decrease of the hopping rate due to the increase of distances between ES chains at the dilution.
O%bo ’ ’
b 0.10 ’ ’ 0.15 ’ ’
0.05
77 K
’ ’0
0.20
0
293 K 0.010
.C
F
r’
.
Q) 0.005
_\
.HH
t
.7 / o--o
L
/(
.
.A001 0.0
--OS
0
1
0.5
10
Concentration, Oh
0
/
./”
Fig. 3. Dependence dilution.
of the ESR line width
of ES in NMP on
CONCLUSION 0.000
I
0.00
0.05
,
I
0.10
*
I
0.15
.
I
0.20
*
!5
Concentration of formic acid, ml / ml Fig. 2. Dependences of the number of electrons per ring, the factor of ESR line shape A/S, the second central moment M2, and the line width AH on added formic acid for ES in NMP at 20’ C. only 0.012 electrons per ring are detected by the ESR method. Low values of e/ring were also found by ESR for other conducting polymers (see, for example, [l, 41) and are explained by coupling of electron spins. According to [5], in metals only 1% of spins are uncoupled. We found that the value of e/ring increases monotonously with addition of acid. Ihe nonmonotonous dependence of the second integral on the degree of polyaniline protonation was found for solution of polyaniline in N,N-dimethylacetamide [6] and for solid polyaniline [7]. It cannot be excluded that in some cases the nomnonotonous dependence may be due to incorrect integration of ESR spectra. As it was pointed above, the measurement of the factor of line shape reveals a broad line superimposed on a narrow one. Ifthe range of integration is not enough wide, we can miss the broad line, and because the ratio
Our data show that ES, in spite of formation of salt bridges, is not rigid. The experiments with powders of ES reveal that a molecular dynamics at room temperatures arises after breaking hydrogen bonds in ES under action of solvents. The experiments with solutions of ES in NMP show that largescale structural rearrangements, both at room temperatures and 77 K, occur at dilution of ES solution; this rearrangement leads to increase of mean distances between paramaguetic centres of ES. REFERENCES 1. Z.H. Wang, AR. MacDiarmid, AG. MacDiarmid, A. J. Epstein, Phys. Rev. B, 43, (1991) no 5. 2. C. Odin, M. Nechtschein, Synth. Met., 43(1991)2943. 3. A.V. Kulikov , Ya.L. Kogan, L.S. Fokeeva, Synth. Met., in press. 4. L.M. Goldenberg et al, Synth. Met., 36(1990) 217. 5. C.A. Wert, RM. Thomson, Physics of Solids, McGraw-Hill Book Company, New York, 1964, chapter 18. 6. V.M. Geskin, Ya. A. Letuchy, Ye. A. Katsman, Synth. Met, 48(1992)241. 7. J.M. Ginder, A.F. Richter, A.G. MacDiarmid, A.J.Epstein, Solid State Communications, 63(1987)97.
Synthetic
Metals
69 (1995) 223-224
Molecular mobility in polyaniline
studied by ESR method
A.V. Kulikov, Ya.L. Kogan, L.S. Fokeeva Institute of Chemical Physics in Chemogolovka, Chemogolovka, Moscow region,
Russian Academy 142432 Russia
of Sciences,
Abstract For emeraldine salts of polyaniline (ES), segmental motions in precipitated ES and large-scale rearrangements of ES globules in solutions of polyaniline in N-methylpyrrolidinone (NMP) were found, in spite of the formation of salt bridges between polyaniline chains. For precipitated ES it was found that the asymmetry of the Dysonian ESR line correlates with macroviscosity of solvent. This observation reveals the important role of segmental motions of polyaniline chains in conductivity of ES at microwave frequeucy. For solutions of ES in NMP the broadening of the ESR lines at room temperature and 77 K was found at dilution of the solutions. This observation shows the increase of the distances between paramagnetic centres of ES with the dilution. INTRODUCTION
parameter
The doped conducting form of polyaniline, emeraldine salt (ES), has many salt bridges and is considered in theory of polyaniline conductivity as a solid with static exchange interactions between neighbour chains (see, for example, [l]). Some experimental observations prove that indeed ES is more rigid than undoped emeraldine base (EB). For instance, EB is dissolved in solvents more easily than ES; slow structural relaxation occurs in polyaniline only when it is kept at a negative potential in the undoped form [2]. Nevertheless, in the present report we show that ES, both in powder and dissolved forms, reveals a substantial molecular dynamics. MATERIALS
AND DTSCUSSION
The ESR spectra of ES powders are single lines asymmetric due to the effect of conductivity of samples (the Dysonian lines). The temperature dependences of the asymmetry
0379~6779/95/$09.50
SSDI
in Fig.
1 [3]. The value
of A/B
1 t
25 t
AND METHODS
ES was synthesised during 5 h at decreased temperature in acetonitrile reaction mixture containing 1 M of aniline and 1 M of HC104 at gradual addition of 1 M @B&)&Or. Precipitates were washed several times by acetonitrile and water, then washed by 0.1 M HC104 (to produce salt form of polyaniline ES) or by 0.1 M NaOH (to produce base form of polyaniline EB), and finally dried at 40’ C. ESR spectra were recorded with a SE/x-2544 Radiopan spectrometer equipped by a temperature controller type 660. The ESR line width, AH, was measured between extreme of the first derivative. The asymmetry parameter, A/B, is the ratio of positive and negative peaks (measured as deviations from the background line) of the first derivative. The number of electrons per ring was calculated from the second integrals of ESR spectra of ES and a standard sample. The factor of line shape A/S is the ratio of amplitude and second integral. RESULTS
3.0
A53 are shown
0
1995 Elsevier Science S.A.
0379-6779(94)02425-X
All rights resewed
Fig. 1. Temperature dependence of the asymmetry parameter of ESR lines for powders of ES in different solvents. (1) dry powder, (2) water, (3) mixture of glycerol and water, 1: 1, (4) 1 M HCl, (5) 0.1 M HC104
depends on conductivity (at microwave frequency, in our case’ 9.2 GHz) and size of samples. We used samples of the same size, ca. 1x1~2 mm. For dry powder (curve 1) A/B=1 at all temperatures, i.e. the conductivity of this sample is small. The wetting of the ES powder in some solvents leads to the increase of the parameter A/B, i.e. to the increase of the conductivity (curves 2 - 5). ln our opinion, the solvents facilitate molecular motions of ES (by breaking hydrogen bonds) that results in more frequent collisions between neighbouring chains and, thus, in hopping polarons and increased conductivity. The sharp changes of A03 at freezing are explained by formation of ice that slows down molecular motions in ES.
224
A. V. KuIikov et al. / Synthetic Metals 69 (1995) 223-224
Ihe results of titration of 1% solution of EB in NMP with formic acid are shown in Fig. 2. The line width is actually constant, therefore the variations of the factor of line shape A/S are explained by the superposition of a broad line, the amplitude of the broad line being maximum at 0.05 ml/ml. The measurements of the second moment contirm the existence of a broad line at low volumes of added formic acid. 0.4
o-o-.
E
A\
0
-0.2
-
.-O-O-O o-o-o-o-o-o+
/o
0.0-O
g
\0.
of the amplitude of the broad line to that of narrow one has maximum at 0.05 ml/ml, we would measure a minimum of the second integral at this concentration of formic acid. The ESR spectrum of solutions of polyaniline in Nmethylpyrrolidinone doped with formic acid at 0.2 ml/ml is a Lorentzian line. The line width increases with dilution both at 77 and 293 K (Fig. 3). In this experiment the concentration of formic acid was maintained constant. The width of the ESR line of ES is determined by the averaging of the hyperline structure by static or dynamic exchange interactions between polarons. We explain the broadening by the decrease of the hopping rate due to the increase of distances between ES chains at the dilution.
O%bo ’ ’
b 0.10 ’ ’ 0.15 ’ ’
0.05
77 K
’ ’0
0.20
0
293 K 0.010
.C
F
r’
.
Q) 0.005
_\
.HH
t
.7 / o--o
L
/(
.
.A001 0.0
--OS
0
1
0.5
10
Concentration, Oh
0
/
./”
Fig. 3. Dependence dilution.
of the ESR line width
of ES in NMP on
CONCLUSION 0.000
I
0.00
0.05
,
I
0.10
*
I
0.15
.
I
0.20
*
!5
Concentration of formic acid, ml / ml Fig. 2. Dependences of the number of electrons per ring, the factor of ESR line shape A/S, the second central moment M2, and the line width AH on added formic acid for ES in NMP at 20’ C. only 0.012 electrons per ring are detected by the ESR method. Low values of e/ring were also found by ESR for other conducting polymers (see, for example, [l, 41) and are explained by coupling of electron spins. According to [5], in metals only 1% of spins are uncoupled. We found that the value of e/ring increases monotonously with addition of acid. Ihe nonmonotonous dependence of the second integral on the degree of polyaniline protonation was found for solution of polyaniline in N,N-dimethylacetamide [6] and for solid polyaniline [7]. It cannot be excluded that in some cases the nomnonotonous dependence may be due to incorrect integration of ESR spectra. As it was pointed above, the measurement of the factor of line shape reveals a broad line superimposed on a narrow one. Ifthe range of integration is not enough wide, we can miss the broad line, and because the ratio
Our data show that ES, in spite of formation of salt bridges, is not rigid. The experiments with powders of ES reveal that a molecular dynamics at room temperatures arises after breaking hydrogen bonds in ES under action of solvents. The experiments with solutions of ES in NMP show that largescale structural rearrangements, both at room temperatures and 77 K, occur at dilution of ES solution; this rearrangement leads to increase of mean distances between paramaguetic centres of ES. REFERENCES 1. Z.H. Wang, AR. MacDiarmid, AG. MacDiarmid, A. J. Epstein, Phys. Rev. B, 43, (1991) no 5. 2. C. Odin, M. Nechtschein, Synth. Met., 43(1991)2943. 3. A.V. Kulikov , Ya.L. Kogan, L.S. Fokeeva, Synth. Met., in press. 4. L.M. Goldenberg et al, Synth. Met., 36(1990) 217. 5. C.A. Wert, RM. Thomson, Physics of Solids, McGraw-Hill Book Company, New York, 1964, chapter 18. 6. V.M. Geskin, Ya. A. Letuchy, Ye. A. Katsman, Synth. Met, 48(1992)241. 7. J.M. Ginder, A.F. Richter, A.G. MacDiarmid, A.J.Epstein, Solid State Communications, 63(1987)97.